Method for subcutaneous suprafascial pedicular internal fixation

ABSTRACT

A method for internal fixation of vertebra of the spine to facilitate graft fusion includes steps for excising the nucleus of an affected disc, preparing a bone graft, instrumenting the vertebrae for fixation, and introducing the bone graft into the resected nuclear space. Disc resection is conducted through two portals through the annulus, with one portal supporting resection instruments and the other supporting a viewing device. The fixation hardware is inserted through small incisions aligned with each pedicle to be instrumented. The hardware includes bone screws, fixation plates, engagement nuts, and linking members. In an important aspect of the method, the fixation plates, engagement nuts and linking members are supported suprafascially but subcutaneously so that the fascia and muscle tissue are not damaged. The bone screw is configured to support the fixation hardware above the fascia. In a further aspect of the invention, a three component dilator system is provided for use during the bone screw implantation steps of the method.

BACKGROUND OF THE INVENTION

The present invention concerns a method for internal fixation ofvertebra of the spine.

It has long been known that internal fixation is an adjunct to fusion,such as a transverse process fusion. In early prior art techniques, asurgeon made an incision in the patient's back and separated tissue andmuscle in order to expose a wide area of the spine in which theprocedure was to take place. The fusion and fixation in one prior artprocess is by grafting bone segments between opposing transverseprocesses of adjacent vertebrae. However, this technique resulted ingross destruction of normal anatomy as well as high blood loss.Moreover, this surgical technique did not completely stabilize thevertebra since there was no direct connection between the vertebralbodies.

In more recent times, a surgical technique known as dowel interbodyfusion has been developed. In this technique, bores are formed in disctissue through either open surgery or percutaneous surgery. A dowel ismade to fit into the bores formed in the disc tissue. In still a furthertechnique, all disc tissue is removed between adjacent vertebrae, aswell as the disc plates. Large surface area bone grafts are then placedwithin the clean space to form a graft between the opposing vertebralbodies. In each of these latter two prior art processes it still remainsnecessary to provide some means for fixation to facilitate fusion of thelarge area bone graft or the dowel to the vertebrae.

Many types of instrumentation for performing spinal fixation are knownin the art. For instance, spine instrumentation developed by Harringtonincorporates a hook and rod configuration. Implantation of theHarrington spinal instrumentation requires subperiosteal stripping ofthe spine to avoid injury to the muscular nerves and vessels. Dissectionof the muscle tissue is also required. In some aspects of the earlyHarrington techniques, the spine was stripped clean of the supraspinousand intraspinous ligaments.

Later developed techniques involve hardware which is placed through theskin and through the muscle into the bone. Some of the fixation hardwareremains outside the body, but is removed after the fusion has beencompleted. Techniques of this sort are characterized by high risk of pintract infection and incisional morbidity.

Thus far, each of the prior art spinal fixation and/or fusion techniqueshave been characterized by excessive invasion into the patients spineand back region. What is needed is a technique which allows for adequatestabilization of the spine, yet decreases the chance of infection aswell as patient morbidity. There is further a need for such a methodwhich permits percutaneous removal of the fixation hardware as anoutpatient procedure after fusion has been completed.

SUMMARY OF THE INVENTION

The present invention contemplates a percutaneous fusion technique usingsubcutaneous suprafascial internal fixation. More particularly, theminimally invasive technique of the present invention permits anteriorfusion of the disc space of the lumbar spine following appropriate discresection and bone grafting. The fixation process is suprafascial, thatis above the muscle fascia, but subcutaneous, that is beneath thesurface of the skin. Thus, none of the muscle tissue is destroyed andthe subcutaneous nature of the procedure greatly decreases the risk ofpin tract secretions or infections, or the potential of osteomyelitis.

In more specific aspects of the invention, the technique contemplatesfirst resecting the intranuclear cavity of a damaged disc, includingablation of the superior and inferior end plates. Bone graft material isprepared for introduction into the vacated disc space. Prior tointroduction of the bone graft into the empty disc nuclear space,fixation instrumentation is implanted. In general, this fixationhardware can include self-tapping cannulated bone screws, fixationplates and linking members for laterally fixing plates on opposite sidesof the spinous process.

In the preferred method, guide wires are inserted bilaterally in linewith and into the pedicle. Pedicle screws are advanced over the guidewire and engaged into a predrilled bore in the pedicle. After the guidewire is removed, the skin is elevated and tissue in the suprafascialsubcutaneous space is dissected to permit insertion of the fixationplates. The appropriate plates are first engaged over the ipsilateralscrews and then the procedure is repeated for the contralateral bonescrews at each level of hardware, that is at each vertebra to bestabilized. The bilateral fixation plates can be laterally connected bydissecting across the midline between corresponding screws and thenpositioning a linking member between the screws using a top-loadinginsertion mechanism. A nut is also top-loaded on to each successivescrew to secure the linking members to the plate and to secure the plateto the pedicle screws.

In the inventive method, the nuts engaging the pedicle screws areinitially loosely threaded onto the screws. The bone screws are thenadvanced into the vertebral body until the hardware resides below thelevel of the skin, but suprafascially in the subcutaneous space at eachlevel of the instrumentation. The nuts are then tightened when theyfixation hardware is in its final resting spot. Once the fixationinstrumentation is in position, the bone graft material is introducedthrough a cannula to the disc space and moved into position by anobturator. With the bone graft in place and the spinal fixation hardwareengaged to the appropriate vertebrae, the subcutaneous tissue is thenirrigated and closed.

In another aspect of the invention, a cannulated fixation or bone screwis provided which is well suited for use with the inventive method. Morespecifically, the screw includes a distal threaded shank and a proximalnut threaded stem which terminates in a driving hub. The distal threadedshank includes self-tapping bone engaging threads. Intermediate thethreaded shank and the stem is a smooth shank of sufficient length sothat only the smooth shank contacts muscle tissue when the fixationinstrumentation is in place. Near the stem end of the smooth shank is amounting hub which supports the fixation plate before the nut is engagedon the threaded stem. The smooth shank preferably accounts for aboutone-half of the length of the screw as measured from the tip of the boneengaging threaded shank to the underside of the mounting hub.

In a further aspect of the invention, a three component dilator systemis provided to facilitate instrumentation of the vertebrae. Inparticular, the dilator system includes three concentrically disposedhollow dilator tubes, each tapered at its respective end for atraumaticintroduction into the patient. Each of the three dilators issuccessively smaller in diameter but larger in length. The intermediateand smallest dilator tubes have knurled ends to grasp for removal duringsteps of the method.

It is one object of the present invention to provide a method forinternal fixation of the spinal column which is minimally invasive andwhich poses a minimal health risk to the patient. Another object is toprovide such a technique which further permits subcutaneous removal ofthe temporarily implanted hardware in an out-patient procedure.

A further object of the invention is realized by the present techniquewhich contemplates subcutaneous but suprafascial fixation to avoiddamage to the spinal musculature and ligaments. Further objects andcertain advantages of the present invention will become apparent fromthe following description of the invention.

DESCRIPTION OF THE DRAWINGS

FIG. 1. is a section view through the spinal column of a patient shownafter implantation of fixation instrumentation using the method of thepresent invention.

FIG. 2 is a side view of a bone screw adapted for use in the method ofthe present invention.

FIG. 3 is a posterior view of the spinal column of the patient afterimplantation of fixation instrumentation using the method of the presentinvention, showing bilateral fixation with linking members across thespinal midline, as viewed beneath the skin but with the muscle tissueremoved to expose details of the underlying vertebrae.

FIGS. 4A-C are side views of the components of a three component dilatorsystem for use with the method of the present invention during steps forimplanting the bone screw into a vertebra.

DESCRIPTION OF THE PREFERRED EMBODIMENT

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiment illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications in the illustrated device, and such further applicationsof the principles of the invention as illustrated therein beingcontemplated as would normally occur to one skilled in the art to whichthe invention relates.

The present invention first contemplates steps for a percutaneous fusiontechnique, such as may be used to fuse adjacent vertebrae after disctissue has been removed. According to the preferred embodiment of theinvention, the method is conducted on lumbar vertebrae, although it isbelieved to be adaptable to other portions of the spine. The patient,after appropriate preparation, is positioned prone on a radiolucentpadded frame which allows for both AP (anterior/posterior) and lateralfluoroscopic visualization during the entire procedure. An initial APfluoroscopic view is taken with an external guide pin placed parallel tothe plane of the end-plates of the affected disc to assure the properorientation of the procedure relative to the disc space. A guideline isdrawn on the AP radiograph along the guidepin image to demarcate theplane of entry.

In the preferred embodiment of the fusion technique of the presentmethod, a bi-portal approach is utilized to clean out the disc nuclearspace. Entry points for local anesthesia are located bilaterally fromthe midline, nominally about ten centimeters bilaterally from theradiograph guideline (varying between 8 centimeters for a smallerpatient to 12 centimeters for a larger patient).

After the fascia and intrafascia musculature are appropriatelyanesthetized, discography is performed using a two-needle techniquebilaterally. Pursuant to the preferred method, the initial needle entrypoint to the disc annulus is located on the mid-pedicle line on the APradiograph, which is defined by the line created by the midportion ofthe pedicle above and below the disc space being instrumented. Bothneedles are advanced into the posterior central portion of the nucleusand triangulation is begun. The discography is performed to confirm thenature of the disc disease and the contained verses the uncontainedcondition of the involved disc structure. (An uncontained disc is a discwhich has ruptured through the posterior longitudinal ligament and/orannulus, which therefore allows a free flow of dye from the intranuclearspace into the epidural space.)

The procedure of the preferred method continues with the introduction ofcannulae into the disc space. The hubs of the discogram needles can beremoved and then serve as guide wires for the cannulae. Dilating probesare place over the guide wires bilaterally into the annulus of the disc.Sheaths are placed over the probes over which progressively largercannulae can then be advanced to dilate the annulus to an appropriatediameter for intradiscal work. Again, this prior procedure is performedbilaterally at the two entry points described above, and is repeated foreach affected disc.

In one aspect of the method, a visualization scope is place through eachcannula to verify the annulus and to confirm the anatomy under thecannula. If no nerve tissue is observed and the annular tissue ispresent, a trephine is introduced after the visualization scope has beenremoved. The annulus is perforated and successively sized trephines areused to open an annular hole of adequate dimension for the purpose ofdisc removal and extra-discal visualization.

Once the trephine operation is complete at each bilateral entry point,two portals are provided. Disc resection can be conducted through oneportal while a verifying scope can be placed at the other portal.Triangulation of the disc material through one portal is consideredachieved when direct visualization of the disc resecting instrumentoccurs through the viewing portal. The intranuclear cavity of theaffected disc is completely resected and the superior and inferiorend-plates are ablated using cutting and sucking instruments or throughthe use of laser-assisted probes. The instruments may be removed fromeach portal and transposed for complete resection of the disc nucleus.Rapid disc removal instruments for the nucleus and rapid burring devicesfor the end-plates can be used to resect the tissues in preparation forfusion. The instruments may also be used to resect the posteriorligamentus structures and the interannular ring to create an adequatecavity for introduction of the bone graft. Acceptable devices includeburrs, laser, curettes and gauges for the ablation of the end-platetissues to the state of bleeding bone. Both rigid and flexible scopescan be used for the verification of the complete resection of theintranuclear cavity. Once the disc material has been completely clearedout of the cavity and the end-plates completely ablated, and obturatoris placed in each cannula to prevent contamination during thissucceeding portions of the procedure.

At this point of the preferred technique of the present invention, bonegraft harvesting is undertaken. The bone graft harvesting can beaccomplished according to any known techniques suitable for thispurpose. In one specific embodiment of the inventive method, afterappropriate anesthesia and analgesia, a small incision is made overeither posterior superior iliac crest to expose the outer crest for boneharvesting. The bone is procured from the corticocancellous table andprepared for the grafting process. The bone graft is fashioned to beaccommodated within the inner diameter of the largest outer cannulaemployed during the disc resection described above. The bone may bemixed with other components, including osteoinductive proteins ormorphogenic materials. The bone harvest cite is then irrigated, driedand closed over a small drain.

The next step of the inventive process, the instrumentation step, occursunder direct fluoroscopy. Under AP fluoroscopic view, a guide wire(preferably a 0.062 guide wire is introduced with the sharp end of theguide wire being inserted into the skin at a position slightly offsetfrom the area to be cannulated. In one specific embodiment, the guidewire is introduced one centimeter lateral to the area to be cannulated.The position of the guide pin is verified by fluoroscopy angled to 15degrees in line with the pedicle. The guide pin is used to palpate thecortex over the pedicle and is then secured by tapping with a mallet toprevent movement of the pin until further advancement is desired. Afterthe guide pin has been locked into the cortex, tissue dilators areapplied to protect the surrounding muscle tissue. The guide pin is thenadvanced, under lateral fluoroscopic view, into the pedicle and withinthe vertebral body. The position of the pin is confirmed using both APand fluoroscopy views. This procedure is repeated for each pedicle o thevertebra. Thus, for each vertebra to be instrumented, a pair of guidepins are positioned at about a 15° angle from the midline and along eachpedicle of the vertebra.

After the guide pin insertion process is complete, an incision is madeat the guide pin insertion site, which, in one specific embodiment, isabout 2.0 cm. in length. Then, using pick-ups and Metzenbaum scissors,subcutaneous tissue is dissected suprafascially. Metzenbaum scissors arealso used to dissect the suprafascial subcutaneous tissues from theipsilateral pin across the midline to the contralateral guide pin.Dissection of this tissue provides space for connection of pediclescrews in subsequent steps of the method.

In the next step of the inventive method, a three component tissuedilator system is used to dilate the tissue at each guide pin to acceptfirst a cannulated drill bit and then a larger diameter cannulatedself-tapping bone screw. The dilator system comprises three taperedtubes of increasing diameter and decreasing length. The tubes areintroduced successively from smallest diameter to largest diameter toprovide adequate access through the tissue for later steps of themethod. After the three component tissue dilator system has beeninserted, the smallest of the internal dilators is removed allowing forinsertion of the drill bit along the guide pin. The bit is used to drillinto the initial one-third of the pedicle. The bit is removed and theintermediate tissue dilator is then removed, leaving the largest dilatorstill in place. The self-tapping bone screw is inserted through thelargest dilator over the guide wire until it is advanced to at least 50%depth of the pedicle. After the position of the bone screw has beenconfirmed by lateral fluoroscopy, the guide wire is removed and thescrew advanced until the proximal tip of the screw is at the level ofthe skin incision. This procedure is repeated for each pedicle in eachsuccessive vertebra until all the bone screws are in place for the finalinternal fixation instrumentation.

In one specific embodiment, the cannulated drill bit has an outerdiameter of 4.5 mm, while the cannulated bone screw can have a diameterbetween 5.5-8.5 mm. Thus, in this specific embodiment, the intermediatedilator of the three component dilator system has an internal diameterof at least 4.5 mm, and preferably 5.2 mm to receive the drill bittherethrough. Likewise, the largest dilator has an internal diameter atleast larger than the bone screw, and preferably 9.6 mm to accommodate arange of bone screw diameters.

Pick-ups are again used to elevate the skin and Metzenbaum scissors areused to dissect any remaining subcutaneous suprafascial tissue asrequired to accommodate insertion of an elongated fixation plate. Anappropriately sized fixation plate is inserted using forceps through the2.0 cm. incisions. It is understood that the fixation plates are sizedto fit over bone screws engaged in the pedicles of adjacent vertebrae toprovide adequate fixation at each side of the spinous process. Thefixation plates for the ipsilateral screws are first inserted by forcepsand then the procedure is repeated for the contralateral side at eachvertebral level requiring instrumentation. In addition, linking membersare inserted through the incision at the ipsilateral guide pin andpassed across the midline in the subcutaneous space to engage the ipsi-and contra-lateral bone screws to accomplish trans-lateral linkage. Thelinking member can be of the type sold by Danek Medical, Inc., assigneeof the present invention, as its CROSSLINK™ product. Once each of thefixation plates and linking members have been engaged over theappropriate bone screws, a nut is applied in a top-loaded fashion toloosely secure the hardware together.

After each nut is initially threaded onto its corresponding screw, thebone screws are advanced as necessary so that all of the fixationhardware lies subcutaneously, but suprafascially, at each level of theinstrumentation. Once each of the bone screws has reached its finalresting place within the vertebra, and once all the instrumentation,including the fixation plates and linking members, is within theappropriate suprafascial subcutaneous space, the nuts are tightened,while the bone screws are held, at each successive level, thus creatinga firm interlock between all of the components of the fixation system.AP and lateral fluoroscopic views can document the final position of thehardware to the satisfaction of the operating surgeon.

Once the fixation instrumentation has been inserted, each bone screwinsertion cite is thoroughly irrigated as well as the subcutaneous spacewhich has accepted the fixation components. The incisions are dried andhemostasis verified followed by closure of the incision withsubcutaneous absorbable sutures.

With the fixation hardware in place, attention is returned to the priorportals through which the disc resection was conducted. In this step ofthe method of the present invention, the obturators are removed from theportals and the previously harvested bone graft material is introducedthrough one cannula into the disc space. A visualizing scope is extendedthrough the cannula in the other portal for confirmation of entry of thebone graft into the disc space. A smooth obturator is inserted into thecannula to facilitate advancement of the bone graft material through thecannula into the empty disc nuclear space. After the ipsilateral portalhas been completely filled with bone graft material, the same procedureis performed at the contralateral portal. Visual verification of thegrafting procedure in the contralateral portal is not possible becausethe first portal has been filled by graft material. However, fluoroscopycan be used to identify the introduction of the obturator into the discnuclear cavity, thereby confirming the location of the bone graftmaterial. Upon completion of the grafting process, the cannulae areremoved, the subcutaneous tissue irrigated and the discography entrypoints are closed with absorbable sutures.

With the foregoing description of the inventive method in mind,attention is directed to the figures. In FIG. 1, a cross-sectional viewof a vertebral region of a patient shows a vertebra 10 having pedicleportions 11. In this superior section view, a disc is shown with itsannulus 15 intact but with an empty nuclear space 16 after the disctissue has been resected. Fixation hardware is shown at only one side ofthe midline ML defined by the spinous process of the vertebra 10.However, as depicted in FIG. 3, fixation instrumentation is implanted oneither side of the midline ML. FIG. 3 further shows fixation betweenadjacent vertebrae, identified as vertebra 10 and 10', withcorresponding body portions 10a, transverse processes 10b and 10c,spinous processes 10d, and laminae 10e.

The entry sites 25a and 25b shown in FIG. 1 are used in the discresection steps of the method. FIGS. 1 and 3 show the location of theportals 26a and 26b through which the disc annulus is removed and thebone graft material introduced. As described above, the entry sites 25aand 25b are nominally 10.0 cm bilaterally from the midline ML. Theportals 26a and 26b are oriented so that the disc resection tools can beinserted below the transverse processes 10c of the vertebra at the levelof the nerve root.

Referring again to FIG. 1, the skin 20 of the patient is shown dissectedfrom the fascia 22 protecting muscle tissue 23 to provide a suprafascialsubcutaneous space 25. An incision 27 is shown through which thefixation hardware is inserted in accordance with the method describedabove. The fixation hardware includes a bone screw 30, which ispreferably a pedicle screw. A fixation plate 40 is mounted on the screw30, held in place by a nut 42. In the posterior view of FIG. 3, thefixation hardware is also shown as including linking members 44 spanningacross the midline between corresponding bone screws 30.

As can be seen most clearly in FIG. 1, the method of the presentinvention provides a technique for instrumenting adjacent vertebra tofacilitate fusion. Implantation of the fixation instrumentationaccording to the inventive method causes minimal invasion to thepatient, with the insertion occurring through a single incision, likeincision 27, aligned with each pedicle. Most significantly, the hardwareresides within the suprafascial subcutaneous space 25 so thatdestruction of muscle tissue is not required. With this method, patientmorbidity rates are reduces, while healing rates are improved. Since thefixation hardware resides above the muscle layer, removal can beconducted in an out-patient procedure under a local anesthetic.

Referring now to FIG. 2, the details of a bone screw particularlyadapted for the present method is shown. The screw 30 includes a distalthreaded shank 31, which in the preferred embodiment is configured as aself-tapping pedicle screw in accordance with known technology. Theproximal end of the screw 30 includes a machine threaded stem 34, whichis threaded for engagement with the nut 42 used to fix the fixationplate 40 and linking member 44. The stem 34 terminates in a driving hexrecess 35 which is engaged by an appropriate screw driving tool as knownin the art. (Alternatively, a hex projection can be used in lieu of therecess 35, with an appropriate change in the screw driving tool.)

Intermediate the distal shank 31 and proximal stem 34 is a smooth shankportion 32. The smooth shank portion 32 defines a hub 33 near theproximal threaded stem 34. The hub 33 includes a surface 33a configuredto support the fixation plate 40. When the nut 42 is tightened onto theproximal stem 34, the fixation plate 40 is locked between the nut andthe hub surface 33a. The hub 33 supports the fixation plate to keep itwithin the suprafascial space 25. The surface 33a is preferably slightlycurved to fit within a scalloped fixation plate of known design in theart. The screw 30 is cannulated along its entire length, as representedby the bore 36 provided for guidewire insertion.

The smooth shank portion 32 provides a non-irritating surface forcontacting the fascia and muscle tissue. The length of the smooth shankportion 32 is determined by the muscle thickness around the instrumentedvertebra, and is generally equal in length to the length of the boneengaging threaded shank 31. In one specific embodiment, the screw 30 hasa length measured from the tip of the bone engaging shank 31 to theunderside of the mounting hub 33 of 65-75 mm. The bone engaging distalshank 31 has a nominal length of 35 mm which provides optimum engagementwith the vertebra. The smooth shank portion 32 accounts for theremainder of the 65-75 mm length, or between 30-40 mm. The machinethreaded stem 34 has a length, as measured from surface 33a of themounting hub 33, that is sufficient to accommodate the fixation plate40, a nut 42 and a linking member 44. In the specific embodiment, thelength of the machine threaded stem 34 is 10-15 mm so that very littleof the stem projects beyond the nut. The bone screw 30 can have adiameter of between 5.5-8.5 mm as required for the patient and fixationprocedure. It is understood, of course, that the specific dimensions areillustrative of a nominal bone screw configuration. These dimensions canbe varied as required for a particular patient or procedure, while stilladhering to the basic concepts of the present invention.

Referring now to FIGS. 4A-C, the components of a three component dilatorsystem 50 are shown. As described above, the dilator system is used tofacilate implantation of the bone screw 30 into the vertebrae of thepatient. The system 50 includes three successively smaller and longerdilator tubes 51, 56 and 61. Each dilator tube is tapered at itsrespective tip 53, 58 and 63 for atraumatic introduction of the tubesthrough the skin and tissue of the patient. Each of the tubes iscannulated or hollow as represented by respective bores 52, 57 and 62therethrough. The bore 52 in the thinnest dilator tube 51 has a diametersufficient to accept a guidewire therethrough. The bore 57 in theintermediate diameter dilator tube 56 has a diameter slightly largerthan the outer diameter of the dilator tube 51. Likewise, the bore 62 inthe largest diameter dilator tube 61 is slightly larger than the outerdiameter of the dilator tube 56.

The ends of the smallest and intermediate diameter tubes 51 and 56, ends54 and 59 respectively, are knurled to provide a gripping surface forremoval of the tubes. The lengths of the tubes are graduated so that thesmallest diameter tube 51 has the greatest length, while theintermediate tube 56 has is longer than the outermost larger diameterdilator tube 61. This length differential also facilitates sequentialremoval of the tubes 51 and 56, just prior to and just after thevertebra has been drilled in the instrumentation step of the method.

In one specific embodiment of the three component dilator system 50 ofthe present invention, the smallest diameter dilator tube 51 has anouter diameter of about 5 mm, a length of 152.5 mm, and a cannulatedbore diameter of about 2 mm. The intermediate dilator tube 56 has anouter diameter of 9.4 mm, a length of about 140.0 mm, and a cannulatedbore diameter of 5.15 mm (leaving 0.15 mm clearance for insertion of thetube 51). The final dilator tube 61, through which the bone screw 30 isinserted, has an outer diameter of 11.1 mm, a length of 127.0 mm and acannulated bore diameter of 9.58 mm to receive the intermediate dilatortube 56, as well as the bone screw 30, therethrough.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiment has been shown and described and that allchanges and modifications that come within the spirit of the inventionare desired to be protected.

What is claimed is:
 1. A method for internal fixation of the spinecomprising the steps of:a) inserting a guide pin into a pedicle of atleast two vertebrae to be instrumented for internal fixation; b) makingan incision in the skin at the entry site for each guide pin; c)dissecting subcutaneous suprafascial tissue between each entry site; d)initially advancing a bone screw over each guide pin through eachincision and into each pedicle, the bone screw having bone engagingthreads and machine threads for engaging a nut; e) elevating the skinbetween each entry site; f) inserting a fixation plate into thesubcutaneous suprafascial space and supporting the plate on and betweenat least two bone screws with the machine threaded portion projectingabove the plate; and g) clamping the fixation plate to the bone screwsusing nuts engaging the machine threaded portions of the bone screwswith the fixation plate supported above the fascia and muscle tissue butbeneath the skin of the patient.
 2. The method for internal fixation ofthe spine of claim 1, further comprising the following steps between theplate insertion and clamping steps:loosely engaging a nut on the nutthreaded portion of each of the bone screws; and further advancing eachbone screw deeper into the respective pedicle until the fixation platelies immediately adjacent the fascia.
 3. The method for internalfixation of the spine of claim 1, wherein:each of the steps (a)-(g) isrepeated on opposite sides of the spinous process for bilateralinstrumentation of each pedicle of each vertebrae.
 4. The method forinternal fixation of the spine of claim 3, further comprising the stepsof:dissecting subcutaneous suprafascial tissue between the ipsilateraland contralateral bone screws at each level of instrumentation of thevertebrae; and after the step of inserting the fixation plate, insertinga linking member between bilateral bone screws at the same level ofinstrumentation with the machine threaded portion of the bone screwsprojecting above the linking member.
 5. The method for internal fixationof the spine of claim 1, wherein the step of advancing the bone screwincludes the steps of:advancing a cannulated drill bit over each guidepin; and using the drill bit, drilling into the pedicle to apredetermined depth less than the length of the bone engaging threads ofthe bone screw.
 6. The method for internal fixation of the spine ofclaim 5, wherein:in the drilling step the predetermined depth is aboutone-third of the pedicle depth; and in the step of initially advancingthe bone screw, the bone screw is advanced into at least one-half of thepedicle depth.
 7. The method for internal fixation of the spine of claim1, wherein the step of advancing the bone screw includes the stepsof:dilating the tissue at each entry site using three dilators havingsuccessively larger diameters; removing the smallest dilator; advancinga cannulated drill bit over each guide pin through an intermediatedilator; and using the drill bit, drilling into the pedicle to apredetermined depth less than the length of the bone engaging threads ofthe bone screw.
 8. The method for internal fixation of the spine ofclaim 7, wherein the step of advancing the bone screw further includesthe subsequent steps of:removing the intermediate dilator, leaving thelargest diameter dilator; and advancing the bone screw over the guidepin and through the largest dilator.